In this scene, Lady Macbeth shows how ambitious she is—perhaps even moreso than her husband. She is so excited and overwrought by the witches' prophecy that Macbeth will be king that she wants to murder Duncan immediately. Her only fears are that Macbeth is too kind-hearted (and therefore not up to the task) and that she herself will be overtaken with compassionate scruples. She wants to kill all the human feeling in both of them, so they can steel themselves to murder a king who has treated them well.
Lady Macbeth calls on dark spirits to help make her cruel and remorseless. She doesn't want any thoughts of heaven to keep her from acting. She calls out to the dark spirits,
Come to my woman’s breasts,And take my milk for gall, you murd'ring ministers . . .
Lady Macbeth also shows that she is prone to act before thinking through the consequences of her actions: it never occurs to her that her conscience might have a mind of its own and come back later to haunt her.
In this scene, Lady Macbeth receives a letter from Macbeth in which he tells her about the prophecies and his recent promotion to Thane of Cawdor. Shakespeare's portrayal of Lady Macbeth in this scene is uniformly negative. Specifically, he presents her as an ambitious and ruthless woman, capable of convincing Macbeth that killing Duncan is the only way to realize his dreams of becoming king.
To demonstrate this, take a look at her dialogue after receiving the letter. She talks about Macbeth lacking the necessary feelings ("the illness") to go for the crown. This is because Macbeth is too kind-hearted ("too full o' th' milk of human kindness.") In contrast, Lady Macbeth urges Macbeth to return home so that she can use her influence to ready him for murder:
Hie thee hither,
That I may pour my spirits in thine ear
And chastise with the valor of my tongue
All that impedes thee from the golden round.
This implies that while Macbeth lacks the cunning and cold-bloodedness to kill, Lady Macbeth has it in abundance and, more importantly, can allay any of his fears about committing such a heinous crime.
Moreover, by using the words "golden round" to describe the crown, Shakespeare shows that Lady Macbeth is just as eager for power as her husband.
Finally, after the servant brings news of Macbeth and Duncan's arrival, Lady Macbeth asks to be unsexed and to be filled with "direst cruelty." In other words, she wants to possess all the traits which make murder possible so that she can guarantee their accession to power and prestige.
In this scene, Shakespeare presents Lady Macbeth as a manipulative, ruthless, and diabolical woman. When she reads Macbeth's letter that acquaints her with his news from the Weird Sisters, she becomes anxious for him to come home. She believes that he is too good and kind to "catch the nearest way" to the throne, and she wants to "pour [her] spirits into [his] ear" and make him see her way.
She also prays to murderous spirits to come to her and remove any sense of remorse or compassion from her and fill her up, from her toes to the top of her head, with ruthlessness and strength so that she can go forward with the evil plans she's already making. Although she desires to be completely remorseless and without feeling, the fact that she has to pray for it makes it seem as though this is not really her natural character. She is ambitious, clearly, but if she were already totally ruthless, she wouldn't have to pray for assistance to be ruthless. Therefore, Shakespeare presents her as manipulative and ambitious, someone who wants to be tougher than, perhaps, she really is.